A glass plate used for a liquid crystal display panel or the like is usually produced by first scribing a large mother glass plate with a wheel cutter so as to form scribe lines, and then cutting the mother glass plate along the scribe lines into pieces of a prescribed size. The wheel cutter includes a circular cutter wheel tip held by a tip holder.
The cutter wheel tip is circular, has an outer circumferential surface, and is formed of a hard metal, sintered diamond or the like. A center of the entire outer circumferential surface projects in such a manner so as to be sharp and acts as a blade edge.
The cutter wheel tip has a hole at an axial center thereof, and a rotating shaft is passed through the hole. The hole has a diameter which is slightly larger than the diameter of the rotating shaft, such that the cutter wheel tip is freely rotatable with respect to the rotating shaft.
Each of two side portions of the rotating shaft are respectively supported by a pair of walls included in the tip holder. The cutter wheel tip is located between the walls. The two side portions of the rotating shaft are inserted into supporting holes of the walls.
FIG. 16 is a cross-sectional view of such a rotating shaft 33 which is supported by the walls. As shown in FIG. 16, each of two side portions (only one is shown) of the rotating shaft 33 is inserted into a supporting hole 34 formed in a wall 31 to be supported. The inner diameter of the supporting hole 34 is slightly larger than the outer diameter of the rotating shaft 33, so that there is a slight tolerance G between an inner circumferential surface of the supporting hole 34 and an outer circumferential surface of the rotating shaft 33.
For scribing a brittle object such as a glass plate or the like, the wheel cutter is moved with respect to the brittle object in a prescribed scribing direction. The cutter wheel tip runs on a surface of the brittle object so as to be pressed thereon. In this state, the wheel cutter keeps on moving on the surface in the prescribed scribing direction.
When the cutter wheel tip runs on the brittle object, a force is applied on the cutter wheel tip and also on the rotating shaft 33 engaged with the cutter wheel tip. When the cutter wheel tip is pressed on the brittle object, the rotating shaft 33 is elevated and pressed on an upper portion of the inner circumferential surface of the supporting hole 34.
The rotating shaft 33 contacts the upper portion of the inner circumferential surface of the supporting hole 34 at one contact line Y. In this state, the entire reaction force from the inner circumferential surface of the supporting hole 34 is applied on the rotating shaft 33 at contact line Y. When impact is applied on the cutter wheel tip, the impact is also applied on the rotating shaft 33 engaged with the cutter wheel tip. Then, a reaction force from the entire inner circumferential surface of the supporting hole 34 is applied on the rotating shaft 33 at contact line Y . As a result, the rotating shaft 33 can be undesirably broken at contact line Y.
When the cutter wheel tip pressed on the surface of the brittle object moves on the surface, a frictional force is applied on the rotating shaft 33 due to a reaction force applied on the rotating shaft 33 from the inner circumferential surface of the supporting hole 34, thus restricting the rotation of the rotating shaft 33. The cutter wheel tip rotates with respect to the rotating shaft 33, and thus the edge of the cutter wheel tip, rotatably engaged with the rotating shaft 34, moves while rotating in contact with the surface of the brittle object. Thus, the surface of the brittle object is scribed.
The conventional cutter wheel having the above-described structure has the following problems. The rotating shaft 33 has substantially the same outer diameter as the inner diameter of the supporting hole 34. Therefore, it is possible that the reaction force applied on the rotating shaft 33 from the inner circumferential surface of the supporting hole 34 is too low to stop the rotation of the rotating shaft 33 with certainty, resulting in the rotating shaft 33 slipping and thus further rotating undesirably. Especially when the pressure contact force between the supporting hole 34 and the rotating shaft 33 changes due to a change in the pressing force of the cutter wheel tip on the surface of the brittle object which occurs during the scribing operation, or due to a change in the scribing speed of the cutter wheel tip, the possibility of the rotating shaft 33 slipping so as to rotate on the brittle object surface increases.
When the rotating shaft 33 slips and rotates, the resistance caused by friction between an outer circumferential surface of the rotating shaft 33 and an inner circumferential surface of the cutter wheel tip having a slightly larger diameter than the diameter of the rotating shaft 33 becomes unstable. This may undesirably result in the quality of the scribe lines being non-uniform.
Since the rotating shaft 33 slips, the outer circumferential surface of the rotating shaft 33 and the inner circumferential surface of the supporting hole 34 are abraded, which may undesirably prevent long and stable use of the wheel cutter.
When the rotating shaft 33 slips, a force is applied on the rotating shaft 33 from the inner circumferential surface of the supporting hole 34 in an abnormal direction, which may undesirably damage the rotating shaft 33.
The present invention, for solving the above-described problems, has an objective of providing a tip holder for realizing long and stable use of a rotating shaft engaged with a cutter wheel tip.
Another objective of the present invention is to provide a tip holder for allowing the cutter wheel tip to stably scribe a surface of a brittle object.